In general, the after-burner systems of turbojets comprise a plurality of flame-holder arms extending radially relative to the axis of the turbojet from an outer casing of an after-burner chamber into the hot primary flow formed by the combustion gas, each flame-holder arm containing a fuel feed duct which extends radially inside the arm from the outer casing.
A burner ring is carried by the downstream portion of the flame-holder arms and extends either in the primary flow in the vicinity of the boundary between the primary flow and the cold secondary flow generated by the fan of the turbojet and flowing around the after-burner chamber, or else in the secondary flow itself. The ring may be made up of sectors whose facing ends are placed substantially end to end and are mounted on the flame-holder arms. These ring sectors are fed with fuel by ducts connected radially to the outer casing and they are also fed with air coming from the secondary flow.
When the burner ring extends in the portion of the after-burner chamber that conveys the primary flow, its air and fuel feed means need to pass through the secondary flow and also through a portion of the primary flow, and said means lead to relatively large head losses in these flows, thereby reducing the efficiency of the turbojet.
Since the temperature of the primary flow in the after-burner chamber can be as high as 2000° K. in after-burn mode, it is necessary to provide the means for feeding fuel to the after-burner with thermal protection, and such thermal protection is liable to further increase head losses in the primary and secondary flows.